Electronic appliances have recently undergone progressive size reductions. For example, a conventional battery pack of a portable telephone had a thickness ranging from 5 mm to 6 mm, but has recently been required to have a thickness ranging 2.5 mm to 4 mm. The electronic appliance is becoming smaller, and its thermal capacity accordingly becomes smaller, and an increase in the speed of heat generation accordingly becomes larger. This situation requires a quick-melting property for thermal fuses used for such protective purpose.
FIG. 5A is a partially cut-away top view of a conventional thermal fuse, and FIG. 5B is a sectional view of the fuse along line 5B—5B in FIG. 5A.
As shown in FIG. 5A and FIG. 5B, the conventional thermal fuse includes a first insulating film 2 having respective leading ends of a pair of metal terminals 1 provided on a top face of the film 2, a fusible alloy 3 provided over the first insulating film 2 and between the leading ends of the metal terminals 1, a second insulating film 4 provided over the fusible alloy 3 and affixed to the first insulating film 2 and metal terminals 1, and metal layers 5, 6 provided on the leading ends of the pair of metal terminals 1 and connected to the fusible alloy 3. The metal layers have larger wettability to the fusible alloy 3 than the metal terminals 1 and first insulating film 2.
The area of the metal layers 5, 6 is S, the length and volume of the fusible alloy 3 are L1 and V, respectively, the distance between the leading ends of the pair of metal terminals 1 is L2, and the distance from the bottom face of the second insulating film 4 to the top face of the metal layers 5, 6 is d.
FIG. 6A and FIG. 6B show the metal terminals 1 which are heated.
First, the fusible alloy 3 is heated to over its melting point and melts, and as shown in FIG. 6A, the fusible metal 3 is then divided into parts (point A in the figure) of the fusible alloy 3. Then, as shown in FIG. 6B, the temperature of the entire thermal fuse exceeds the melting point of the fusible alloy 3, and the fusible alloy 3 melts. Then, the melting fusible alloy 3 moves onto the metal layers 5, 6 having a large wettability connected to the metal terminals 1. As a result, a volume V(L1+L2)/2L1 including a volume V(L2/L1) between the metal terminals 1 and a volume V(L1−L2)/2L1 on the metal layers 5, 6 out of the volume V of the fusible alloy 3 moves onto the metal layers 5, 6.
As batteries become smaller, it is necessary for the thermal fuse to be smaller and thinner.
In order to reduce the size and thickness of the conventional thermal fuse, the fusible alloy 3 may have its size reduced. Accordingly, the fusible alloy 3 generates heat by its resistance due to an increase of a current passing the alloy, and melts down by the heat. Hence, the fusible alloy 3 cannot have the reduced size. The distance L2 between the leading ends of the metal terminals 1 cannot be reduced too much in order to ensure cut off of the current during the operation of the thermal fuse. As a result, in the conventional thermal fuse, since a volume Sd enclosed by the metal layers 5, 6 and the second insulating film 4 is small, the volume V(L1+L2)/2L1 of the fusible alloy 3 moving to the metal layer 5 or the metal layer 6 exceeds the volume Sd. Then, as shown in FIG. 6B, the fusible alloy 3 overflows to the metal terminals 1 or first insulating film 2 from above the metal layers 5, 6. In this case, since the wettability of the metal terminals 1 and first insulating film 2 on the fusible alloy 3 is smaller than that of the metal layers 5, 6, the fusible alloy 3 moves slowly during its melt-down, and the separation of the fusible alloy 3 during the melt-down is delayed, that is, the thermal fuse does not melt down quickly.